First of all, I DON'T want to use planters, because that would be expensive, plus they're tedious and annoying.

Trying to get this to work in a world where I want it to be perpetually winter. Obviously trying to grow plants outside in winter won't work, so how the heck can I grow them indoors? I tried to construct something that would work as a greenhouse, but the game won't let me garden indoors...

And my dude is a warlock (though the game still calls him a "witch", ugh), who practices alchemy, so gardening is kinda a must.

Plant outside (you can make it temporarily summer using cheats to do this if you have to, I guess -- not sure if Seasons will let you plant in winter), and then move the plants indoors using moveobjects. That's how there are plants in tombs in WA.

Pretty sure you still need dirt floor though.

Haven't tested this lately, but it used to work

Alternatively, and I haven't tested this either, I suppose you could plant outside and then build around the plants.

If you don't get gardening interactions using these methods, you could also try removing PART of the roof. I read somewhere recently that there's some roof percentage required to block weather, like 60% of the room covered with roof, and some HIGHER roof percentage required to block gardening, like 80%. Not sure of these numbers though. But the idea was that there's a sweet spot where weather doesn't come in but you can still garden.